1. Field of the Invention
The present invention relates to the manufacturing of semiconductor components, and more specifically to the deposition by epitaxy of a silicon layer on a single crystal silicon substrate.
2. Discussion of the Related Art
Generally, in the field of semiconductor component manufacturing, it is known to grow epitaxial layers of a determined type of conductivity and level of doping on substrates of a determined type of conductivity, generally different from that of the epitaxial layer, and of a determined doping level. The epitaxy process obtains thin single crystal layers having a constant doping level across their thickness, conversely to methods implying a dopant diffusion step, in which the doping level decreases with the diffusion depth.
Vapor phase epitaxy methods, which have been developed for many years, operate very satisfactorily when the doping level of the substrate is homogenous and relatively low. However, in modern technologies, buried layers, that is, heavily-doped localized layers of the first and/or of the second type of conductivity formed under the epitaxial layer are used more and more. The buried layers are obtained by first performing a high dose implantation of a dopant chosen in the substrate before performing the epitaxial growth. These layers are, for example, meant to form bipolar transistor collectors and must be very heavily doped. This results in a known drawback, currently called autodoping, which is that the implanted doping has a tendency, during the epitaxy process, to dope the epitaxial layer above and beside the implanted region. The epitaxied layer no longer has the desired resistivity. This disadvantage is becomes more significant as the epitaxial layer is made thinner since the autodoping phenomenon essentially occurs in the initial growth phase of this layer. Further, it obviously appears that the autodoping rate of the epitaxial layer depends on the surface of the doped areas meant to become buried layers. The larger this surface, the more the autodoping. Thus, from one circuit to another, according to whether more or less buried layers have been made, the epitaxial layer will not have the same general resistivity. This phenomenon is difficult to master. It goes against what is sought in any industrial method, that is, to obtain controlled and repeatable results, and which are, if possible, identical for different products of a given technology.
At the present time, many solutions have been brought forward to attempt to solve autodoping problems and more specifically to decrease arsenic autodoping without increasing boron autodoping upon deposition of a lightly-doped epitaxial layer on a substrate containing arsenic implanted areas and boron implanted areas.
It has, in particular, been proposed to reduce the pressure in the epitaxy reactor, to increase the duration of some thermal steps, especially the anneal step before the actual epitaxy, and to choose pressure conditions adapted to reducing the growth rate. These methods have yielded more or less satisfactory results according to the operating conditions but none has, up to now, been generally acclaimed.
It has also been proposed to perform an epitaxy in several steps but no satisfactory solution has been provided and such a method is not used industrially. This may be due to the fact that, in an epitaxy, a very large number of parameters have to be set and optimized, among which are the pressure, the temperature, the duration of each step, the shiftings between steps, the compositions of the gases, etc.
Further, if some of the solutions brought forward have reduced the autodoping due to arsenic, they have worsened the autodoping due to other dopants such as boron and the general result has thus not been satisfactory.